docs: update secret manager documentation

[diegomarquezp]

This PR explains the introduction of the sm@ syntax introduced by #3411

From its description:
We will introduce support for resolving values of the form sm@secret_id in addition to the already existing sm://secret_id format. Users should move away from sm:// and use sm@ instead. Usage of sm:// will raise a warning about this being removed in the future. This has been tested using spring-framework:6.2.2-SNAPSHOT.

The introduction of this syntax was suggested in a discussion of a regression caused by spring-framework:6.2.1 - see spring-projects/spring-framework#34124

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[zhumin8]

LGTM. Do you mind adding some context to the pr description? So if we come back to it and forget, we still have a reminder why this recommendation was made. If you got any links from Spring for recommending this new syntax will be great.

[diegomarquezp]

LGTM. Do you mind adding some context to the pr description? So if we come back to it and forget, we still have a reminder why this recommendation was made. If you got any links from Spring for recommending this new syntax will be great.

@zhumin8 thanks for the review!
I have modified the description explaining how the new support for Spring Cloud 2024 has most of the context, plus the discussion with the spring team where they suggest this syntax.